Recovery of phosphorus from sludge



Dec. 10, 1963 c, BARBER L 3,113,839

RECOVERY OF PHOSPHORUS FROM SLUDGE Original Fiied June 19, 1961 sSheets-Sheet 1 PHOSPHORUS CONDENSER WATER T0 CONDENSER S PRAYS ym gm 90471 IN VEN TORS.

Dec. 10, 1963 J. c. BARBER ETAL RECOVERY OF PHOSPHORUS FROM SLUDGE 3Sheets-Sheet 2 Original Filed June 19. 1961 .rzuummm mu M 1 ENT R g .2NV 0 5 United States Patent Ofilice 3,113,839 Patented Dec. 10, 19633,113,839 REQOVERY OF PHGEWHORUS FROM SLUDGE James C. Barber, George H.Megar, and Thomas S. Sloan, Florence, Ala, assignors to Tennessee ValleyAuthority, a corporation of the United fit-ates Griginai appiicationdune 19, 1961, Ser. No. 11%,204, now Patent No. 3,084,629, dated Apr. 2,1963. Divided and this application Feb. 19, 1962, Ser. No. 174,325)

4 tClairnS. (Ci. 23--165) (Granted under Title 35, US. Code (1952), see.266) The invention herein described may be manufactured and used by orfor the Government for governmental purposes without the payment to usof any royalty therefor.

This application is a division of our copending application, Serial No.118,204, filed June 19, 1961, now US. Patent No. 3,084,029, for Recoveryof Phosphorus From Sludge.

Our invention relates to an improvement in the production and recoveryof phosphorus, and more particularly to an improved process for treatingphosphorus sludge formed during the production of phosphorus by smeltingphosphate rock.

Heretofore it has been the practice in the chemical industry to produceelemental phosphorus from phosphate rock by reducing the phosphate rockwith coke or other carbonaceous reducing agents in the presence ofsilica. Such a process is normally carried out in equipment such as anelectric phosphorus smelting furnace in which the phosphorus vaportherefrom carries with it such foreign matter as particles of rock,sand, reducing agent, and fluorine compounds. The result of such processis that when the phosphorus vapor is condensed and collected under Waterin order to recover the phosphorus some of the product upon removal fromthe condenser is found to be of the character of a sludge containingvarious concentrations of phosphorus as well as the above-mentionedimpurities and water.

It has been the practice to install electrostatic precipitators at thephosphorus furnaces for the purpose of removing the above-mentionedsolid impurities from the gases prior to the condensation of phosphorus.However, the precipitators have been found to remove only about 60 to 90percent of the solids in the gas. The gases are then cooled with waterspray to condense the phosphorus vapor to liquid phosphorus. The solidimpurities that remain in the gas after passing through theelectrostatic precipitators are also collected in the water-phosphorusmixture that drains from the condenser. in the condenser sump (vesselinto which the condenser is drained) a layer of relatively high-gradephosphorus is obtained at the bottom;

above this layer is a mixture referred to as sludge whichconsists ofphosphorus droplets or globules, solid impurities, and water; and abovethe second layer is a water layer containing phosphorus droplets andsolids in suspension. The boundary between the sludge and the watersuspension is not clearly defined.

The sludge is viscous and sometimes sticky; it consolidates duringstorage and becomes more viscous. It is very difficult to pump and burnconsolidated sludge in conventional burner assemblies. Also, the acidproduced by burning the sludge is contaminated.

Various methods of preparation and recovery of the phosphorus from thesludge have been suggested in the prior art; among these are filtration,distillation, and briquetting methods.

Phosphorus in the sludge can be separated from its impurities bydistillation, but this method of separation has several disadvantageswhen practiced on a commercial scale. Phosphorus losses are high as aresult of the formation of red phosphorus which remains in the residue,

and some phosphorus is lost as uncondensed vapor. Also, investment costsfor a distillation assembly are high.

Gur invention is directed to an improved process for recoveringphosphorus from sludge by means utilizing the introduction of smallamounts of dispersing agents into the sludge to be treated. We havediscovered that the use of as little as 1.5 parts of our dispersingagent per 1,060 parts by weight of sludge substantially reduces thestickiness of the sludge, and the viscosity of the treated sludge isreduced so as to result in a material which is sufiiciently fluid as tobe readily pumped to and burned in conventional phosphorus burners torecover, as P 0 the phosphorus values from said sludge.

We have found that the use of our dispersing agent increased the pH ofthe sludge from a value of about 3.0 up to a value of about 6.0, andthat the amount of dispersing agent used is highly critical in thatexcessive use thereof causes the pH of the slurry to rise to above 7.0with the resulting formation of phosphine gas.

It is therefore an object of the present invention to provide animproved method of treating phosphorus sludge by a process in whichsubstantially all of the elemental or uncombined phosphorus contained inthe sludge can be removed in a simple and economical manner.

Still another object of the present invention is to provide a method oftreating the phosphorus sludge whereby a substantial part of anycombined phosphorus contained in the sludge can be recovered as P 0 Afurther object of the present is to provide an improved method forrecovering phosphorus from sludge in which the sludge is consolidatedand is treated with a small but critical amount of dispersant to reduceits viscosity and make it readily pumpable to ensure ease in burningsame to recover therefrom the phosphorus values.

A still further object of the present invention is to provide animproved method for recovering phosphorus from sludge in which thesludge is consolidated and is treated with a small but critical amountof dispersant to reduce its viscosity and make it readily pumpable, andin which the cost of treating the sludge with dispersant is in the rangeof about 30 cents to 60 cents per ton.

A still further object of the present invention is to provide animproved method for recovering phosphorus from sludge in which thesludge is consolidated and is treated with a small but critical amountof dispersant to reduce its viscosity and make it readily pumpable, inwhich the cost of treating the sludge with dispersant is in the range ofabout 30 cents to 60 cents per ton, and in which substantially largeramounts of low-cost reducing agents (bituminous coal) may be used in thephosphorus smelting furnaces.

Another object of the present invention is to provide an improved methodfor the recovery of phosphorus from sludge in which method phosphorusparticles in the sludge are caused to coalesce by means of appliedcentrifugal force with the resulting recovery of elemental phosphorustherefrom.

Still another object of the present invention is to provide an improvedmethod for the recovery of phosphorus from sludge in which method thephosphorus particles in the sludge are caused to coalesce by means ofapplied centrifugal force with the resulting recovery of elementalphosphorus therefrom, and in which the impurities removed from thesludge may be utilized as a binder to agglomerate furnace feedmaterials.

In carrying out the objects of the present invention in one formthereof, we employ a method of effectively decreasing both the viscosityand the stickiness of the sludge through the use of an extremely smallamount of effective dispersing agent selected from a group ofcommercially available lignosulfonates. In addition, we have alsodiscovered that the use of sodium hydroxide as the dispersing agentprovides results which indicate that it is almost as effective as thelignosulfonate agents, and that, furthermore, the use of such sodiumhydroxide material as a dispersing agent may prove to be somewhat moreeconomical than the use of a lignosulfonate dispersant.

Our invention, together with further objects and advantages thereof,will be better understood from consideration of the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIGURE 1 is a diagrammatical illustration of a phosphorus condenser sumpshowing how the phosphorussludge and the water-suspension layers arecollected in the sump.

FIGURE 2 is a diagrammatical illustration of the preferred embodiment ofthe present arrangement for fluidizing sludge with our dispersing agent.

FIGURE 3 is a diagrammatical illustration showing an alternative methodof recovering phosphorus by means of centrifuging the sludge to causecoalescence of the phosphorus particles therein and to permit recoveryof clemental phosphorus as a high-grade material.

Referring now more specifically to FIGURE 1, there is shown a condensersump 1 in which a layer of relatively high-grade phosphorus is obtainedas bottom layer 2. Above bottom layer 2 is shown a layer of the sludge 3which consists of phosphorus droplets or globules, solid impurities, andWater. Above layer 3 is shown water layer 4 containing phosphorusdroplets and solids in suspension.

Microscopic examination of the sludge shows that it consists of globularparticles of yellow phosphorus, 1 to 2 millimeters in diameter down to afew microns in size. Fine particulate foreign matter is associated withthe phosphorus. There is little tendency for the phosphorus particles tocoalesce; however, some of the smaller globules agglomerate intoclusters. Electrical charges and physical barriers of foreign solids arethought to retard or prevent coalescence of this phosphorus.

The solids in the sludge result from the precipitation of solids in thecondensing system and particles in the furnace charge being carried overinto the condensing system. Following is the approximate composition ofthe solid fractions of the sludge: 32 percent P 23 percent F, 8 percent(3210, and 7 percent SiQg. The amounts of P 0 and F in the sludge in onesystem were found to be about 2.1 and 1.6 .tons per day, respectively.

When bituminous coal is used as a reducing agent in the phosphorusfurnaces, some of the carbonaceous material in the coal is volatilizedand carried over to the condenser. The carbonaceous material collects inthe sludge layer and causes the sludge to be sticky and more diflicultto handle. Bituminous coal is much less costly than coke (which isnormally used as the reducing agent); however, the amount of coal thatcan be used is limited because of the adverse effect on the sludgeproperties. We have found that the amount of low-volatile bituminouscoal used in the furnaces is limited to about half of the total reducingagent requirement because of the adverse elfect of the carbonaceousmaterial on the handling properties of the sludge. The development ofsaid method to cope with the sludge problem results in greatereconornies in the smelting furnaces by allowing the use of largeramounts of bituminous coal or the use of lower cost (highvolatile-matter content) coal.

The sludge collected in con-denser sump 1 may be pumped to an acid unitand burned, or the material may be pumped to storage tanks for later usein the production of phosphoric acid. When the sludge is burned shortlyafter its collection, some of the impurities enter the acid and cause itto be contaminated. This problem is of greater concern when bituminouscoal is used in the furnaces, because the carbonaceous material is notcompletely burned and carbon sometimes collects in the acid. Inaddition, fluorine is evolved when the sludge is burned,

and the fluorine causes serious corrosion problems in the acid unit.

On the other hand, if the sludge is stored for a time before it isburned, some of the phosphorus slowly collects as a liquid layer in thebottom of the sump, as is shown in FIGURE 1; consequently, thephosphorus content in the sludge layer decreases. Upon prolonged storagethe sludge layer consolidates and becomes extremely viscous. Thephosphorus content decreases so much that it may be difiicult orimpossible to burn the material in an acid unit. The presence ofcarbonaceous material causes the sludge to be sticky and furthercomplicates its pumping, mixing, and burning.

Referring now more specifically to FIGURE 2, sodium hydroxide solutionis fed through line 11 and means for control 12 into storage tank 11Mixing of the contents of tank .13 is effected by recirculation withpump 14. It has been found that the contents of tank 13 can be mixed byrecirculation with pump 14 in a convenient manner; however, mixing canalso be accomplished, perhaps more quickly, by the use of a mechanicalagitator, not shown. Internal steam coils 15 are used as a means forheating the contents in tank 13. The sodium hydroxide solutiondispersing agent is added slowly over a period of about 2 hours untilthe pH of the sludge in the tank increases to about 6.0. The sludge isthen further mixed for approximately an additional 5 or 6 days, and thenmay be stored for an additional period of time prior to burning in aphosphoric acid unit.

Referring now more specifically to FIGURE 3, there is shown analternative method of coalescing phosphorus globules in the sludge andseparating liquid phosphorus from its impurities by centrifugal means.Sludge in compartment 29 is pumped by means 21 through strainers 2-2 and23 through sludge meter 24-, and into disk-type centrifuge 25 via line26. Hot water from water heater 27 is led through line 23, through meter29, and introcluced into centrifuge 25. Underflow from centrifuge 25 isfed via line 33 into underflow tank 31 and then .to storage tank 32. Theunderflow is a liquid material containing more than about percentphosphorus. Overflow is fed by line 33 into overflow tank 34 and then byline 35 into settling tank as. The overflow is a slurry containing solidimpurities and carbonaceous material originally present in the sludge.The phosphorus content of the overflow material is usually less thanabout 3 percent. The phosphorus recovered in the underflow and collectedas a layer in storage tank 32 is suitable for use in any subsequentfertilizer process.

In order that those skilled in the art may better understand how thepresent invention can be practiced the following examples are given byway of illustration and not by way of limitation.

EXAMPLE I Phosphorus was produced in an electric furnace from a mixtureof phosphate rock, coke, and silica. Impure phosphorus collected in thecondenser sump was pumped to storage tanks. Phosphorus settled out ofthe impure phosphorus mixture and was pumped off, leaving a consolidatedviscous sludge. The sludge in the different storage tanks containedvarious phosphorus contents and viscosities depending on the storagetime and the amount of phosphorus that had settled out. The viscositywas as great as 15,000 centipoises, and the phosphorus content was aslow as 7 percent. One storage tank contained 52 tons of consolidatedsludge which had a viscosity of 3,400 centipo-ises and a pH value of 3,but the sludge was not sticky. Phosphorus content of the sludge was 6-9percent at the bottom, 44 percent in the middle, and 14 percent at thetop of the sludge layer. Piping was installed in the tank to permitmixing by recirculation of the contents from one end of the tank to theother, as is shown in FIGURE 2. The mixing of the contents of the tankby recirculation with a pump was a convenient mixing method, but perhapsthe mixing could have been accomplished much more quickly by use of amechanical agitator. The contents of tie tank were heated by means ofinternal steam coils to a temperature of 140 F. A percent sodiumhydroxide solution was added slowly (a total of 150 pounds of NaOH wasadded over a period of about 2 hours), after which the pH of the sludgewas found to have increased to about 6.0. Exccsses of NaOl-l (pl-Ivalues above 7.0) would cause hazardous phosphine gas to form. Thesludge was mixed 72. hours by pumping from one end of the tank to theother, after which time analysis showed relatively uniform phosphoruscontent. After storage for 3 days, the sludge was burned in a phosphoricacid unit to produce superphosphoric acid. Data on the acid-plantoperation are given in Table 1.

Table 1 OPERATING DATA FOR ACID UNIT WHILE BURNING FLUIDIZED SLUDGEEqnivalent to a phosphorus burning rate of 2,500 pounds per hour.

EXAMPLE II Phosphorus was produced in an electric furnace by thereduction of phosphate rock with a mixture of coke and medium-volatilebituminous coal as reducing agent, and silica as a flux. Phosphoruscollected in the condenser sump (as is shown in FIGURE 1) as a yellowliquid layer in the bottom of the sump. A sludge layer was found on topof the liquid phosphorus layer. The sludge contained carbonaceousmaterial that resulted from the use of bituminous coal as a reducingagent in the furnace; consequently, the sludge was a sticky, viscousmaterial. The sludge was pumped to the storage tank and, in the storagetank, consolidated to form a sticky, viscous, black mass which was verydifiicult to pump and which would not mix with phosphorus. Thestickiness of the sludge was measured by inserting a glass rod 2 /2inches into the material and Weighing the amount of material adhering tothe glass rod. Such a measurement showed that 2.8 grams of materialstuck to the rod at a temperature of 150 F. The stickiness of the sludgeprevented an accurate measurement of the viscosity in the usual manner,the sludge being somewhat more viscous than material that had nocarbonaceous content. The phosphorus content of the sludge was 34percent near the bottom of the layer and 31 percent near the top. The pHwas 2.6. A 25 percent sodium hydroxide solution was added to the sludge,and the contents of the tank were mixed "as indicated in FIGURE 2. About180 pounds of NaOH was added to about tons of sludge in the tank. Thestickiness of the sludge, as measured by the glassrod method, was Zeroafter N aOH addition and mixing. The material was sufficiently fluid tobe readily pumped and burned.

EXAMPLE III The following example follows the alternative embodiment ofthe present invention, employing the use of centrifuging apparatus.Phosphorus was produced in an electric furnace by the reduction ofphosphate rock with a mixture of coke and low volatile bituminous coal.Phosphorus in the sludge was present as globules mixed with solidparticles, carbonaceous material, and water. The phosphorus content ofthe sludge was 38 percent. The sludge was centrifuged in the assembly asshown in FIGURE 3. This assem'bly consisted of strainers, a sludgemeter, disk-type centrifuge, hot water heater, and necessary pipes andpumps. The sludge feed rate was 3.5 gallons per minute, and hot Water(190 F.) was added to the sludge at a rate of 1.5 gallons per minute.The mixture of sludge and hot water flowed through strainers havingopenings equivalent to 40 mesh. Additional hot water (0.5 gallon perminute) was used as wash water in the centrifuge. The centrifugeoperated so as to impart a centrifugal force of 5,200 times gravity tothe sludgewater mixture. The centrifugal force caused the phos phorusglobules in the sludge emulsion to coalesce to liquid phosphorus havinga greater density than the impurities in the sludge, and be dischargedas underfiow from the centrifuge. Underfl-ow rate was 1.2 gallons perminute, and the overflow rate Was 4.3 gallons per minute. Underfiowcontained 92 percent phosphorus (93 percent of the total phosphorus inthe feed). Phosphorus content of the overflow was 2.7 percent (7 percentof the total phosphorus in feed). Phosphorus recovered in the underflowwas suitable for use in any of the fertilizer processes. Overflow was aslurry containing the solid impurities and carbonaceous materialoriginally in the sludge. The settled solids obtained as overflow fromthe centrifuge were used as an experimental binder to agglomerate amixture of calcined phosphate, coke, and silica. (Mixture consisted of69.8 percent minus S-mesh nodule fines, 9.2 percent minus A-inch plusS-mesh coke, and 11.0 percent building sand.) When 10 percent of thesolids-carbonaceous material mixture was added to thephosphate-cokesilica mixture and briquetted at a pressure of 5,000pounds per square inch, cylindrical briquets 1 /2- inches in diameter,which had a crushing strength of pounds, were formed in a laboratorypress. After heating the briquets in an atmosphere of CO at 1050 F., thestrength of the briquets was 820 pounds. Thus, we have discovered thatthe sticky carbonaceous material in the sludge resulting from the use ofbituminous coal as a reducing agent in phosphorus furnaces may berecovered by centrifuging, and this impurity may be utilized as a binderfor agglomerating phosphorus-furnace charge. Or, the impurities in thesludge may be returned to the raw-material process, the carbonaceousmaterial burned out, and the P 0 value recovered as feed for thephosphorus furnaces.

EXAMPLE IV Aqueous efiiuent from the phosphorus condensers containingsolids and globules of phosphorus was fed to the centrifuge assemblyshown in FIGURE 3 at a rate of 7.8 gallons per minute. Wash water wasadded at a rate of 2.0 gallons per minute. centrifuging of this efiluentin a manner similar to that described in Example 111 caused thesuspended phosphorus particles to coalesce to liquid phosphorus. Thecentrifuging forces developed by the centrifuge caused almost completecoalescence of the phosphorus particles and resulted in the recovery of99.9 percent of the phosphorus in the centrifuge feed. The recoveredphosphorus collected as underflow from the centrifuge contained 64percent phosphorus, dry basis.

EXAMPLE V Consolidated sludge was treated with a dispersing agent todecrease viscosity and stickiness according to Example I above. Thetreated sludge was then centrifuged to recover the phosphorus inaccordance with the process shown in Example III above. Treatment withthe dispersant permitted the sludge to be more readily pumped andhandled. Such a treatment with dispersant facilitates the centrifugingoperation, and particularly facilitates the straining of the sludgeprior to centrifuging.

While We have shown and described particular embodiments of ourinvention, modifications and variations thereof will occur to thoseskilled in the art. We Wish it to be understood, therefore, that theappended claims are intended to cover such modifications and variationswhich are within the true scope and spirit of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In the method of producing phosphorus by smelting phosphate rock Witha reducing agent in the presence of silica and thereafter condensing,collecting, and recovering the resulting phosphorus vapor under waterand separating accumulated phosphorus-bearing sludge from the relativelypure elemental phosphorus resulting from the condensation of thephosphorus vapor, the improvement which comprises adding sodium[hydroxide to said sludge as a dispersing agent in quantity sufiicien-tto raise the pH of said sludge from about 3.0 to about 6.0 agitating theresulting mixture for a period of about 2 to 6 days; thereafter burningthe mixture; and recovering the resulting phosphorus oxide vapor.

2. In the method 10f producing phosphorus by smelting phosphate rockwith a reducing agent in the presence of silica and thereaftercondensing, collecting, and recovering the resulting phosphorus vaporunder water and separating accumulated phosphorus-hearing sludge fromthe relatively pure elemental phosphorus resulting from the condensationof the phosphorus vapor, the improvement which comprises adding lessthan about 2 parts of sodium hydroxide per 1,000 parts of sludge byweight to said sludge, said sodium hydroxide addition sufficient toraise the pH of said sludge to about 6.0; agitating the resultingmixture for a period of about 2 to 6 days; thereafter burning themixture; and recovering the resulting phos phorus oxide vapor.

3. In the method of producing phosphorus by smelting phosphate rock witha reducing agent in the presence of silica and thereafter condensing,collecting, and recovering the resulting phosphorus vapor under waterand separating accumulated phosphorus-bearing sludge from the relativelypure elemental phosphorus resulting from the condensation of thephosphorus vapor, the improvement which comprises adding less than about2 parts of sodium hydroxide per 1,000 pants [of sludge by Weight to saidsludge, said sodium hydroxide addition sufiicient to raise the pH ofsaid sludge to about 6.0; agitating the resulting mixture; coalescingthe phosphorus particles in said sludge per se by application of, incentrifugal means, centrifugal forces thereto; removing from saidcentrifugal means as overflow a Water slurry containing solids andcarbonaceous materials; and recovering from said cen trifugal means asunderflow, liquid phosphorus.

4. In the method of producing phosphorus by smelting phosphate rock witha reducing agent in the presence of silica and thereafter condensing,collecting, and rec-overing the resulting phosphorus vapor under Waterand separating accumulated phosphorus-bearing sludge from the relativelypure elemental phosphorus resulting from the condensation of thephosphorus vapor, the improvement which comprises (adding less thanabout 2 parts of sodium hydroxide per 1,000 parts of sludge by weight tosaid sludge, said sodium hydroxide addition sufficient to raise the pHof said sludge to about 6.0; agitating the resulting mixture; coalescingthe phosphorus particles in said sludge per se by application of, incentrifugal means, centrifugal forces thereto; removing from saidcentrifugal means as overflow containing less than about 7 percentphosphorus by weight, a Water slurry containing solids and carbonaceousmaterials; and recovering from said centrifugal means liquid underflowcontaining more than about 93 percent phosphorus by weight.

References Cited in the file of this patent UNITED STATES PATENTS417,943 Readman Dec. 24, 1889 452,821 Wing May 26, 1891 1,334,474Waggaman Mar. 23, 1920 1,788,838 Lang Jan. 13, 1931 2,039,297 Curtis May5, 1936 2,135,486 Almond Nov. 8, 1938 2,267,077 Burke Dec. 31, 19412,302,956 Retalliau Nov. 24, 1942 2,744,866 Kahler May 8, 1956

1. IN THE METHOD OF PRODUCING PHOSPHORUS BY SMELTING PHOSPHATE ROCK WITHA REDUCING AGENT IN THE PRESENCE OF SILICA AND THEREAFTER CONDENSING,COLLECTING, AND RECOVERING THE RESULTING PHOSPHORUS VAPOR UNDER WATERAND SEPARATING ACCUMULATED PHOSPHORUS-BEARING SLUDGE FROM THE RELATIVELYPURE ELEMENTAL PHOSPHORUS RESULTING FROM THE CONDENSATION OF THEPHOSPHORUS VAPOR, THE IMPROVEMENT WHICH COMPRISES ADDING SODIUMHYDROXIDE TO SAID SLUDGE AS A DISPERSING AGENT IN QUANTITY SUFFICIENT TORAISE THE PH OF SAID SLUDGE FROM ABOUT 3.0 TO ABOUT 6.0 AGITATING THERESULTING MIXTURE FOR A PERIOD OF ABOUT 2 TO 6 DAYS; THEREAFTER BURNINGTHE MIXTURE; AND RECOVERING THE RESULTING PHOSPHORUS OXIDE VAPOR.